Trinuclear nickel and cobalt complexes containing unsymmetrical tripodal tetradentate ligands: syntheses, structural, magnetic, and catalytic properties

The coordination chemistries of the tetradentate N2O2-type ligands N-(2-pyridylmethyl) iminodiethanol (H(2)pmide) and N-(2-pyridylmethyl) iminodiisopropanol (H(2)pmidip) have been investigated with nickel(II) and cobalt(II/III) ions. Three novel complexes prepared and characterized are [(Hpmide)(2)Ni-3(CH3COO)(4)] (1), [(Hpmide)(2)Co-3(CH3COO)(4)] (2), and [(pmidip)(2)Co-3(CH3COO)(4)] (3). In 1 and 2, two terminal nickel(II)/cobalt(II) units are coordinated to one Hpmide(-) and two CH3CO2-. The terminal units are each connected to a central nickel(II)/cobalt(II) cation through one oxygen atom of Hpmide(-) and two oxygen atoms of acetate ions, giving rise to nickel(II) and cobalt(II) trinuclear complexes, respectively. Trinuclear complexes 1 and 2 are isomorphous. In 3, two terminal cobalt(III) units are coordinated to pmidip(2-) and two CH3CO2-. The terminal units are each linked to a central cobalt(II) cation through two oxygen atoms of pmidip(2-) and one oxygen atom of a bidentate acetate ion, resulting in a linear trinuclear mixed-valence cobalt complex. 1 shows a weak ferromagnetic interaction with the ethoxo and acetato groups between the nickel(II) ions (g = 2.24, J = 2.35 cm(-1)). However, 2 indicates a weak antiferromagnetic coupling with the ethoxo and acetato groups between the cobalt(II) ions (g = 2.37, J = -0.5 cm(-1)). Additionally, 3 behaves as a paramagnetic cobalt(II) monomer, due to the diamagnetic cobalt(III) ions in the terminal units (g = 2.53, vertical bar D vertical bar= 36.0 cm(-1)). No catalytic activity was observed in 1. However, 2 and 3 showed significant catalytic activities toward various olefins with modest to good yields. 3 was slightly less efficient toward olefin epoxidation reaction than 2. Also 2 was used for terminal olefin oxidation reaction and was oxidised to the corresponding epoxides in moderate yields (34-75%) with conversions ranging from 47-100%. The cobalt complexes 2 and 3 promoted the O-O bond cleavage to similar to 75% heterolysis and similar to 25% homolysis.